The present invention relates to gas turbine combustors and, in particular, to improvements in gas turbine combustors capable of reduced nitrous oxides (NOx) emissions.
In an effort to reduce the amount of NOx in the exhaust gas of a gas turbine, a dual stage, dual mode combustor was developed by the assignee of this invention and disclosed in U.S. Pat. No. 4,292,801. In the aforementioned patent, it is disclosed that the amount of exhaust NOx can be greatly reduced, as compared with a conventional single stage, single fuel nozzle combustor, if there are two combustion chambers established such that, under conditions of normal operating load, the upstream primary combustion chamber performs as a premix chamber, while actual combustion occurs in the downstream secondary combustion chamber. Under this described operating condition, there is no flame in the primary chamber, and its premixed fuel and air is burned in the secondary chamber resulting in a decrease in the formation of NOx. In this condition, the secondary or center nozzle provides the flame source for the operation of the combustor. The specific configuration disclosed in the patent includes an annular array of primary nozzles, each of which discharges fuel into the primary combustion chamber, and a central secondary nozzle which discharges fuel into the second combustion chamber. In other words, the second stage centerbody acts as a pilot for premixed fuel and air passing from the first stage. In a later related development, which is the subject of U.S. Pat. No. 4,982,570 (also commonly assigned), the single central nozzle was replaced by a combined diffusion and premix nozzle which reduces fuel flow to the central diffusion flame from 20 percent of the total fuel flow to about 2 percent of the total fuel flow for the entire combustor. In this later development, the pilot is comprised of a small diffusion sub-pilot and premixed fuel and air, both fed from a secondary stage nozzle within the centerbody.
During premixed combustion, it is desirable for the flame from the second stage pilot to be stable at all times since flame instabilities result in high oscillatory (dynamic) pressure levels. These dynamic pressures may disturb combustor operations by shifting the flame into supposedly non-flame holding regions, i.e., they may create flashback. A more stable premixed pilot at all operating conditions would reduce dynamic pressures and improve the operating characteristics of the combustor and gas turbine.
The existing centerbody cup arrangements shown in the aforementioned U.S. Pat. Nos. 4,982,570 and 4,292,801 (both of which are incorporated herein by reference), generally include cylindrical centerbody cups with an air swirler around the outside diameter of the cup. In the '570 patent, the centerbody cup is shown, in one embodiment, to flare outwardly downstream of the secondary nozzle, but continues to utilize a swirler downstream of the diffusion and premix nozzle and surrounding the centerbody cup wall. The outer swirler columnates the flame for stability, but at the same time shields the flame from other flame holding devices. If the centerbody configuration is not properly designed, high dynamic pressure oscillations may occur at some operating conditions when the secondary nozzle supplying premixed fuel and air in the centerbody is operating. Since the secondary nozzle that supplies premixed fuel and air to the centerbody (as opposed to a more stable diffusion pilot secondary nozzle) is necessary to achieve low emissions levels, a modification of the centerbody in accordance with this invention, is the best method to produce a more stable pilot flame and achieve lower dynamic pressure levels.
The principal objective of this invention, therefore, is to alter the behavior (velocity and direction) of the premixed fuel and air passing through the centerbody cup to pilot the flame. In the original geometry, as described in the two above identified patents, the cup is cylindrical and the flow passing through it is kept that way by an outside layer of swirling air. The modification in accordance with the present invention is to expand the cup diameter into a diverging section, and to eliminate the outer air swirler. This greatly reduces the outer layer of air, and lowers the flow velocity (an equal mass flow through a larger area), while allowing for some radial dispersion of the fuel and air mixture as it leaves the centerbody cup, thereby creating a larger recirculation zone and thus a more stable flame. The diverging cup can also serve as a support for attaching flame holders. Thus, in accordance with this invention, the unique feature is primarily that flame stabilization is achieved without swirl air surrounding the flame. In the present configuration, the diverging centerbody cup is attached to a device which provides a swirled fuel/air mixture to the cup. The mixture burns as the flame is held in the cup, but is allowed to expand radially by the diverging cup as it progresses downstream. The result is a larger recirculation zone with the flame held closer to the cup, which in turn protects the flame, and thereby increases flame stability. At the same time, in place of the previously employed air swirler, a coolant manifold is located between the inner and outer cylindrical portions of the centerbody, diverting a small portion of the air flow through the centerbody to cool the exterior surface of the centerbody cup.
Thus, in accordance with each of the exemplary embodiments of the invention, a centerbody is provided which comprises a pair of cylindrical sections, the outer section being cooled or uncooled. A swirler, which interfaces with the discharge end of the diffusion and premix nozzle, is attached to the inner cylindrical section so as to provide a swirled fuel/air mixture to the cup during operation. Downstream of this inner swirler, a diverging cup portion is added to the inner cylindrical section. The diverging cup portion may have either a straight cone configuration, a curved conical surface or a variation or combination of the two.
The centerbody cup is cooled by impingement and/or film cooling air which passes through a coolant manifold located radially between the inner and outer cylindrical sections, and specifically between the centerbody cup and the outer cylindrical section.
In one embodiment, the diverging cup portion includes diverging end portions connected by a substantially cylindrical intermediate center portion. In this embodiment, the coolant manifold is located radially between and spaced from the centerbody cup and adjacent outer cylindrical section such that a rearward portion extends parallel to the upstream diverging portion of the centerbody cup, and a forward portion extends substantially parallel to the cylindrical intermediate portion of the centerbody cup. The forward end of the manifold remains substantially cylindrical, and thereby intersects the forward diverging end portion of the centerbody cup. At the same time, the rearward end of the manifold is secured to the centerbody cup by a closed annular wall. At a location intermediate the ends of the manifold, an annular wall extends between the coolant manifold and the outer cylindrical portion of the centerbody.
The manifold is provided with a first plurality of apertures upstream of the intermediate connecting wall and a second plurality of apertures downstream of the intermediate connecting wall while the latter is also provided with a third plurality of apertures. By this arrangement, air flowing axially in the annular space between the inner and outer sections of the centerbody will be caused to flow principally from the annular space through the first plurality of apertures in the manifold to a cooling chamber where the air impinges on the exterior surface of the centerbody cup. The air will then exit the cooling chamber through the second plurality of apertures (downstream of the intermediate connecting wall) and return to the space between the manifold and the outer cylindrical section for axial flow out of the centerbody thus providing film cooling. At the same time, a lesser portion of the air will flow directly through the third plurality of apertures in the intermediate connecting wall, bypassing the cooling chamber.
The exact number of apertures in each of the respective first, second and third plurality of chambers may be adjusted as desired to obtain the desired cooling, air flow, etc.
In a second exemplary embodiment, the centerbody cup has a uniformly conical shape, with a coolant manifold attached at its forward end. The manifold extends parallel to the cup, and axially beyond where it intersects the forward end of the outer cylindrical section which, unlike the first described embodiment, is substantially cylindrical along its entire length. In this arrangement, the connection between the forward end of the manifold and the outer cylindrical section may be closed, thus forcing all air to flow through a first plurality of apertures in the manifold to an open-ended cooling chamber, thus cooling the centerbody cup. Alternatively, an annular array of holes may be provided where the manifold intersects the outer cylindrical section so that at least some of the air can flow out of the space between the inner and outer centerbody sections without impinging on the inner centerbody cup. In this second exemplary embodiment, the outer cylindrical section may be film cooled along its entire length, providing additional flow paths out of the annular space between the centerbody inner and outer sections. Variations of the above described embodiments are also disclosed further herein.
In its broader aspects, therefore, the present invention relates to a centerbody construction for use in a gas turbine combustor comprising a pair of inner and outer generally cylindrical members, the inner member having a diverging cup portion attached to its forward end extending toward the outer cylindrical member, and a coolant manifold arranged between the inner and outer cylindrical members radially adjacent the centerbody cup portion.
The centerbody cup constructions of this invention have a number of advantages over the prior, known constructions. By eliminating the outer swirler and utilizing a diverging centerbody cup, the air/fuel mixture burns in the cup but expands radially as the flame progresses downstream. This creates a larger recirculation zone, with the flame held closer to the cup, thereby increasing flame stability. At the same time, a cooling arrangement for the centerbody cup is provided which also creates a barrier to flashback.
Other objects and advantages of the invention will become apparent from the detailed description which follows.